This invention relates to an A.C. powered smoke detector with a back-up battery and, more particularly, to such a detector with a supervision circuit for monitoring the power capacity of the back-up battery without the need for first physically disconnecting the A.C. power from the detector.
There are basically three different types of commercially available smoke detectors categorized according to their electrical power supply. There are first those which are solely battery powered, secondly, those which are solely powered by conventional A.C. power provided by utility companies and, thirdly, those which are A.C. powered but have a back-up battery. The third type of detector is intended to be primarily powered by A.C. with the back-up battery only providing necessary power to operate the detection and alarm circuitry in the event of loss of A.C. power.
It is known to monitor the battery in the first and third types of detectors. In the third type of detector, this is done by operational testing of the battery by simulating a detection signal to the detection and alarm circuitry in response to actuation of a manually actuatable test switch. This procedure requires manual interaction which may not necessarily be forthcoming on a regular basis. It also causes the alarm to sound which depletes the battery to an undesirable extent for test purposes and can desensitize people to the alarm, if tested too often. In addition, failure of the alarm to sound during the test does not necessarily indicate that the battery is defective, for failure of the other parts of the circuitry can also cause such failure.
In the A.C. powered detectors with a back-up battery, the back-up battery voltage can only be compared to a minimum voltage if the A.C. power is first disconnected from the detection and comparison circuitry, and a test switch is manually actuated to perform an operational test as described above with respect to the battery operated detectors.
Accordingly, there has been a need for a battery back-up type of A.C. powered detector with a supervision circuit which monitors the power capacity of the battery, as opposed to only the voltage; and, which performs this monitoring fuction automatically on a periodic basis while the A.C. power remains physically connected to the detector.